Microelectromechanical device provided with an anti-stiction structure, and corresponding anti-stiction method

ABSTRACT

An embodiment of a microelectromechanical device having a first structural element, a second structural element, which is mobile with respect to the first structural element, and an elastic supporting structure, which extends between the first and second structural elements to enable a relative movement between the first and second structural elements. The microelectromechanical device moreover possesses an anti-stiction structure, which includes at least one flexible element, which is fixed only with respect to the first structural element and, in a condition of rest, is set at a first distance from the second structural element. The anti-stiction structure is designed to generate a repulsive force between the first and second structural elements in the case of relative movement by an amount greater than the first distance.

PRIORITY CLAIM

The present application claims the benefit of Italian Patent ApplicationSerial No.: TO2008A000714, filed Sep. 30, 2008, which application isincorporated herein by reference in its entirety.

TECHNICAL FIELD

An embodiment of the present invention relates to amicroelectromechanical device provided with an anti-stiction structureand to the corresponding anti-stiction method.

BACKGROUND

As is known, a microelectromechanical device (MEMS) is constituted byone or more mobile structures provided on a substrate and frequentlyequipped with an actuator and a guide that regulates movement thereof.In general, there are three types of actuators: a first type enablesmovement in a direction parallel to the substrate; a second type enablesmovement in a direction perpendicular to the substrate; whilst a thirdtype enables a rotary movement within a specific range of angles.

A significant defect, which arises in particular conditions in the MEMSdevices considered, is the adhesion (stiction) of the mobile structuresto a fixed element adjacent thereto, or directly to the substrate. It isclear that said phenomenon can lead to serious consequences, even to thepoint of rendering the MEMS systems affected thereby inoperative in anunforeseeable way.

The phenomenon of stiction, in MEMS systems, is generated by the surfaceforces that are exerted between the surfaces of two bodies that are incontact. Of course, the more extensive the area of contact, the greaterthe force of interaction between the surfaces. In addition, furtherfactors that intervene in the phenomenon of stiction, are, among otherthings, the roughness of the surfaces, their degree of wear, the levelof humidity and the temperature of the environment in which themicroelectromechanical structures operate.

Techniques currently used for reduction of the phenomena of stiction inMEMS structures are based upon the reduction of the surfaces of contactand upon low levels of humidity, thus creating conditions that areunfavorable to the occurrence of phenomena of stiction.

However, during use, MEMS structures of a mobile type may come intocontact with further surrounding MEMS structures of a fixed type, forexample, involuntarily on account of shock. Continuous contacts betweenMEMS structures can be the cause of a progressive degradation both ofthe surface of contact of the mobile structures and of the surface ofcontact of the surrounding fixed structures. The formation of particlesof material, that occurs following upon the continuous impacts betweenthe surfaces, is itself a further cause of stiction. Consequently, ithappens that, in these cases, the mobile structures may adhere to thefixed structures, thus jeopardizing their functionality.

SUMMARY

An embodiment of the present invention is a microelectromechanicaldevice and a corresponding method that overcome the drawbacks of theknown art.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of one or more embodiments of the presentinvention, embodiments thereof are now described, purely by way ofnon-limiting example, with reference to the annexed drawings, wherein:

FIGS. 1A-1C show a mechanical model of a MEMS device provided with ananti-stiction structure according to an embodiment of the presentinvention, in three operating steps;

FIG. 2 shows a mechanical model of a MEMS device provided with ananti-stiction structure according to another embodiment of the presentinvention;

FIG. 3 shows a mechanical model of a MEMS device provided with ananti-stiction structure according to a further embodiment of the presentinvention;

FIGS. 4A-4C show simplified top-plan views of a possible implementationof the MEMS device of FIGS. 1A-1C, in the same three operating steps;

FIG. 5 shows a simplified perspective view of a possible implementationof the MEMS device of FIG. 2;

FIGS. 6A-6C show side views of the MEMS device of FIG. 5 duringsuccessive operating steps;

FIG. 7 shows a simplified perspective view of a different implementationof the MEMS device of FIG. 2; and

FIG. 8 shows an overall block diagram of an embodiment of an electronicapparatus incorporating the MEMS device described.

DETAILED DESCRIPTION

FIGS. 1A-1C are schematic illustrations of a mechanical model of anembodiment of an anti-stiction structure 1 for a microelectromechanical(MEMS) device, defined hereinafter simply as device 100, during threeoperating steps.

In detail, the device 100 is represented schematically in its maincomponents and comprises: a first structural element; a secondstructural element; an elastic supporting structure set between thefirst and second structural elements and enabling a relative movementbetween them; and an anti-stiction structure and an arrest structure,which are connected to the first structural element or to the secondstructural element. In particular, the first structural element isformed here by a mobile mass 10, the second structural element is formedhere by a load-bearing structure 6, the elastic supporting structurecomprises one or more suspension springs 12, the anti-stiction structurecomprises at least one flexible element 2, and the arrest structurecomprises a stop element 5.

The suspension springs 12 (just one of which is illustrated) have thefunction of enabling movement of the mobile mass 10 only in pre-setdirections. In the example of FIGS. 1A-1C, the suspension springs 12,which have an elastic constant K_(sm), enable movement of the mobilemass 10 only in a direction u.

The flexible element 2 is anchored to the load-bearing structure 6 andis provided with a resting portion 3. In particular, the flexibleelement 2 is of an elastic type, with an elastic constant K_(f) greaterthan that of the suspension spring 12, for example 10-1000 timesgreater.

The stop element 5 is formed by a rigid structure, for example, by aprojection of the load-bearing structure 6, and has the function oflimiting the movements of the mobile mass 10 and of the correspondingsuspension spring or springs 12 and thus preventing any undesirablefailure. The stop element 5 is anchored to the load-bearing structure 6and provided with a contrast surface 7.

The load-bearing structure 6 may, for example, be a substrate on whichthe anti-stiction structure 1 is provided, an intermediate elementbetween the substrate and the mobile mass 10, or any other structuralelement. Furthermore, the flexible element 2, the stop element 5, andthe mobile mass 10 may be carried by different portions of theload-bearing structure 6.

In conditions of rest, the mobile mass 10 is set at a distance from theflexible element 2 and the stop element 5.

FIG. 1A shows the device 100 in conditions of rest, i.e., in the absenceof external forces F_(u) acting in the direction u on the mobile mass 10(F_(u)=0). In said condition, we shall assume that the mobile mass 10 isset at a first distance I₁ from the flexible element 2 and at a seconddistance I₂ from the stop element 5, with I₁<I₂.

When an external force F_(u)>0 acts in the direction u on the mobilemass 10, the latter undergoes a displacement with consequent reductionof the distances I₁ and I₂. In this step, the flexible element 2 doesnot intervene, and hence does not modify the characteristics ofstiffness and hence of sensitivity of the structure, set in the designstage, by appropriately sizing the elements of the device and inparticular the suspension springs 12.

When the mobile mass 10 displaces by a distance greater than I₁ but lessthan I₂ (FIG. 1B), it initially comes into contact with the restingportion 3 of the flexible element 2, then causes bending of the flexibleelement 2 itself, which generates a braking force that opposes thefurther movement of the mobile mass 10. Since the flexible element 2 iselastic, in the impact the surfaces in contact of the mobile mass 10 andthe flexible element 2 do not degrade, or in any case degrade in aconsiderably reduced way with respect to what would occur in the case ofdirect impact with a rigid element, with a very low constant ofelasticity, for example with the stop element 5.

Even though the external force F_(u) is sufficiently high to bring themobile mass 10 into contact with the stop element 5 (FIG. 1C), onaccount of the braking force, the impact is considerably reduced,consequently reducing the degradation of the mobile mass 10 and of thestop element 5.

It may in any case happen that, following upon an intense use of thedevice 100, the surfaces of contact of the mobile mass 10 and of thestop element 5 wear out, with the consequent formation of a deposit ofparticles of material, and generation of phenomena of stiction. Inpractice, a force of stiction F_(ad) is set up.

However, the flexible element 2 exerts on the mobile mass 10 a repulsiveforce F_(r) of opposite sign with respect to the force of stictionF_(ad). Furthermore, also the suspension spring 12 exerts a force F_(sm)that is of opposite sign to the force of stiction F_(ad).

The total repulsive force F_(rep) is consequently given by the followingformula:F _(rep) =K _(sm) ·I ₂ +K _(f)·(I ₂ −I ₁).

When the external force F_(u) is removed from the mobile mass 10, theforces acting on the mobile mass 10 are the repulsive force F_(rep) andthe force of stiction F_(ad). By appropriately sizing the device 100, itis possible to cause the repulsive force F_(rep) to be always greaterthan the force of stiction F_(ad) so as to guarantee always separationof the mobile mass 10 from the stop element 5.

FIG. 2 shows a different embodiment of the anti-stiction structure 1.

In this case, the flexible element 2 is set fixed with respect to themobile mass 10, whilst the resting portion 3 has the function of pointof contact with the load-bearing structure 6. In this case, I₁ is thedistance between the resting portion 3 and the load-bearing structure 6,but operation is altogether similar to what has been describedpreviously.

In FIG. 3, the arrest element 5 is formed on the mobile mass 10, and thecontrast surface 7 has the function of point of contact with theload-bearing structure 6. Otherwise, the structure is the same as thatof FIG. 2.

FIGS. 4A-4C show, in top-plan view, a possible implementation of theanti-stiction structure 1 of FIGS. 1A-1C, for example applied to themicroelectromechanical gyroscope described in the patent application No.EP-A-1 1677 073 (U.S. Pat. No. 7,258,008), which are incorporated byreference, in which the load-bearing structure 6 comprises a substrate(just one surface 6 a of which is visible) and a frame 6 b of arectangular shape, and the mobile mass 10 is suspended above the surface6 a via elastic springs 12 carried by the frame 6 b.

In particular, FIGS. 4A-4C show three successive operating conditions ofthe anti-stiction structure 1, corresponding to FIGS. 1A-1C,respectively. According to this embodiment, the flexible element 2 isprovided by a beam element, for example made of monocrystalline orpolycrystalline silicon, having one end anchored to the load-bearingstructure 6 and the resting portion 3 free to move in a plane xy. Thestop element 5 is formed by a projection of the load-bearing structure 6extending towards the mobile mass 10.

The mobile mass 10 is typically set in the same plane xy as the frame 6b and is mobile in the plane xy, ideally in the direction y.

In conditions of rest, when an external force F_(y) acting on the mobilemass 10 is equal to zero (FIG. 4A), the mobile mass 10 is set at adistance from the flexible element 2 and from the stop element 5, and,consequently, the flexible element 2 is at rest.

When an external force F_(y) different from zero acts on the mobile mass2, the suspension spring 12 bends, and the mobile mass 10 comes intocontact with the flexible element 2, but, initially, not with the stopelement 5 (FIG. 4B). If the force is sufficiently high, the mobile mass10 in its movement generates a bending of the flexible element 2 andcomes into contact with the stop element 5, which arrests motion thereof(FIG. 4C). As already explained, the flexible element 2 generates inthis step a repulsive force F_(rep) that opposes the further movement ofthe mobile mass, reducing the speed of impact thereof against the stopelement 5.

As soon as the external force F_(y) terminates, the repulsive forceF_(rep) generated by the flexible element 2 co-operates with the forcegenerated by the suspension spring 12 to bring the mobile mass 10 backinto the state of rest, overcoming the force of stiction F_(ad) and thuspreventing stiction of the mobile mass 10 to the stop element 5.

FIG. 5 shows a further embodiment of an anti-stiction structure 1, whichmay be used with a mobile mass 10 that moves perpendicularly or withrotary movement with respect to the load-bearing structure 6, formedhere by a substrate.

Here, the flexible element 2, with an elongated shape, is fixed to themobile mass 10 and precisely is surrounded by the mobile mass 10 itself,from which it is separated by a trench 23, obtained using micromachiningtechniques of a known type.

In detail, the trench 23 is T-shaped, with a first portion 23 aextending in a transverse direction and from a freely oscillable side ofthe mobile mass 10 and a second portion 23 b extending in a directiontransverse to the first portion 23 a. The flexible element 2 extendsalong the first portion 23 a of the trench 23 and is connected to themobile mass 10 via a second torsional spring 22 extending along thesecond portion 23 b of the trench 23.

The flexible element 2 has a projecting portion formed here by a bump 20extending from a free end 2 a of the flexible element 2, in a directiontransverse to the plane of the mobile mass 10, towards the substrate 6.In practice, the bump 20 can be constituted by a portion of the flexibleelement 2 having a thickness greater than that of the mobile mass 10.

The mobile mass 10 is set at a distance from the substrate 6 and issupported by means of first torsional springs 26 that enable a rotarymovement thereof about an axis of rotation 21.

In a condition of rest (FIG. 6A), when no external force acts on themobile mass 10 (F_(z)=0), the mobile mass 10 is substantially parallelto the substrate 6.

In the presence of a high force F_(z), the mobile mass 10 turns aboutthe axis of rotation 21 until the bump 20 is brought into contact withthe substrate 6 (FIG. 6B).

For sufficiently high external forces F_(z), the flexible element 2bends, generating a braking force on the mobile mass 10, until themobile mass 10 comes into direct contact with the substrate 6 (FIG. 6C).More precisely, just one edge 25 of the mobile mass 10 comes intocontact with the substrate 6; in this case, the surface of the substrate6 facing the edge 25 constitutes the stop element 5.

The impact between the mobile mass 10 and the substrate 6 is reducedthanks to the action of the flexible element 2, which reduces thepossibility of damage and/or wear to the parts that come into contact.

Also in this case, the continuous use of the anti-stiction structure ofFIGS. 4 and 5 can cause wear of the edge 25 of the mobile mass 10 and ofthe substrate 6 that are in contact with one another, favoring theoccurrence of phenomena of stiction. However, also in this case, theflexible element 2 generates a repulsive force F_(rep) that contributesto bringing the mobile mass 10 back into the position illustrated inFIG. 6A.

FIG. 7 shows an alternative embodiment of the flexible element 2 usablewith a mobile mass 10 of the type illustrated in FIGS. 5 and 6A-6C.

According to this embodiment, the flexible element 2 has a projectingportion 2 b extending as a prolongation of the flexible element 2itself, beyond the perimeter of the mobile mass 10, and being henceintegral with the flexible element 2. In practice, the flexible element2 has a total length d such as to enable it to project beyond the edge25. In this way, during rotation about the axis of rotation 21, theprojecting portion 2 b of the flexible element 2 comes into contact withthe substrate 6 before the edge 25, behaving in a way substantiallysimilar to the bump 20 of FIG. 5.

Consequently, the anti-stiction structure 1 described enablesimprovement of the behavior of a generic device 100 in regard to thephenomenon of stiction, limiting the force of contact between two mobilebodies with respect to one another, in particular between a mobile massand a load-bearing element, during use of the device 100 or on accountof undesired accidental shock. In this way, the wear of the surfacesthat come into contact, and the consequent stiction, may be considerablyreduced.

Finally, it is clear that modifications and variations may be made tothe anti-stiction structure 1 described and illustrated herein, withoutthereby departing from the spirit and scope of the present disclosure.

For example, a number of anti-stiction structures 1 may be present for asingle mobile mass 10, set on opposite sides of the mobile mass 10, forexample in a symmetrical way. In this way, since the mobile mass 10 maymove in opposite senses along the same direction (in the plane of theload-bearing structure 6 or perpendicularly thereto), i.e., oscillate inopposite senses, the phenomenon of stiction for either sense of movementmay be reduced.

Likewise, in the example of embodiment of FIG. 5, it is possible toprovide bumps 20 extending from the top side of the flexible element 2so as to reduce stiction of the mobile mass 10 in both directions ofrotation, clockwise and counterclockwise, in the case where this were tobecome necessary.

Furthermore, the load-bearing structure 6 may be any fixed or mobileelement, with respect to which the mobile mass 10 moves and with respectto which it is desired to reduce stiction of the mobile mass 10.

The mobile mass 10 may be provided in the same structural layer of theflexible element 2, as illustrated, or else in a different structurallayer.

The stop element 5 may be provided on the structural element 6 and/or onthe mobile mass 10, and the mobile mass 10 may form part of MEMS devicesof a different type, such as accelerometers, gyroscopes, sensors,micromotors, and the like.

For example, the device 100 may be particularly advantageous for use inan electronic apparatus or system 200 (FIG. 8), of a portable type, forexample a cellphone, a PDA, a palm-top or portable computer, an digitalaudio player, a remote control, a video or photographic camera, etc.,comprising a microelectromechanical device 100 of the type describedpreviously; a biasing circuit 222, designed to supply electrical biasingquantities to the microelectromechanical device 100 (in a way in itselfknown and for this reason not described in detail); an interface circuit224, designed to interface with the microelectromechanical device 100for reading one or more electrical quantities associated therewith (in away in itself known and for this reason not described in detail); and amicroprocessor control unit 225, connected to the interface circuit 224,and designed to superintend general operation of the electronicapparatus 200.

Naturally, in order to satisfy local and specific requirements, a personskilled in the art may apply to the embodiments described above manymodifications and alterations. Particularly, although one or moreembodiments have been described with a certain degree of particularity,it should be understood that various omissions, substitutions, andchanges in the form and details as well as other embodiments arepossible. Moreover, it is expressly intended that specific elementsand/or method steps described in connection with any disclosedembodiment may be incorporated in any other embodiment as a generalmatter of design choice.

1. A microelectromechanical device comprising: a first structuralelement; a second structural element rotatably coupled to said firststructural element; an elastic supporting structure extending betweensaid first and second structural elements and configured to enable arotational movement of said second structural element with respect tothe first structural element; and an anti-stiction structure includingat least one flexible element fixed to only the first structural elementand, in a rest condition thereof, arranged at a first distance from thesecond structural element, said anti-stiction structure being configuredto generate a repulsive force between the first and the secondstructural elements in the case of a relative movement of an amountgreater than said first distance, wherein: the first structural elementcomprises a mobile mass, and the second structural element comprises aload-bearing structure, or vice versa; said load-bearing structurecomprises a substrate, said movable mass being suspended above a surfaceof said substrate and being rotatably movable with respect to saidsurface, said flexible element having an elongated shape with a firstend fixed to said movable mass and a second end provided with aprotruding portion facing and arranged at said first distance from saidsurface; and said mobile mass has a trench facing a side of the mobilemass and said flexible element extends within said trench.
 2. Themicroelectromechanical device according to claim 1, further comprising astop element, of a substantially rigid type, fixed only to one of saidfirst and second structural elements wherein, in a rest condition ofsaid device, said stop element being arranged at a second distance fromanother structural element of said first and second structural elements,said second distance being less than said first distance, and beingconfigured to limit the relative movement between said first and secondstructural elements.
 3. The microelectromechanical device according toclaim 2, wherein said stop element is distinct from said flexibleelement and directly faces an abutment portion of said anotherstructural element between said first and second structural elements. 4.The microelectromechanical device according to claim 1, wherein saidflexible element has an elongated shape extending transversely to adirection of movement of the mobile mass and is provided with an endforming an abutment surface facing a portion of said substrate.
 5. Themicroelectromechanical device according to claim 1, wherein saidprotruding portion comprises a bump extending transversely to saidsurface.
 6. The microelectromechanical device according to claim 1,wherein said protruding portion extends as a prolongation of theflexible element, beyond said side of the mobile mass.
 7. Themicroelectromechanical device according to claim 1, wherein the elasticsupporting structure includes a torsional spring coupled to andextending between the first and second structural elements.
 8. Anapparatus, comprising: a substrate; a mass suspended above a surface ofsaid substrate and being rotatably movable with respect to said surface,the mass having a trench facing a side of the mobile mass; a firstelastic member having a first end coupled to the substrate and having asecond end coupled to the mass, the first elastic member beingconfigured to enable a rotational movement of the mass with respect tothe substrate; and an anti-stiction structure including an anti-stictionelement coupled to the mass and, in a rest condition thereof, arrangedat a first distance from the substrate, said anti-stiction structurebeing configured to generate a repulsive force between the mass andsubstrate in the case of a relative movement of an amount greater thansaid first distance, said anti-stiction element having an elongatedshape extending in the trench of the mass and having a first end coupledto said movable mass.
 9. The apparatus of claim 8 wherein the substratecomprises a semiconductor substrate.
 10. The apparatus of claim 8,wherein the first elastic member comprises a spring.
 11. The apparatusof claim 8 wherein the anti-stiction element includes a protuberanceextending toward the surface of the substrate from a surface of theanti-stiction element that faces the surface of the substrate.
 12. Theapparatus of claim 8 wherein the trench is T-shaped and has a firstportion extending transversely from the side of the mass and a secondportion extending transversely from the first portion, the anti-stictionelement extends in the first portion, the anti-stiction further includesa torsional spring coupled to the mass, extending in the second portion,and having a central portion coupled to the anti-stiction element. 13.The apparatus of claim 8, further comprising: a mass stop extending fromthe surface of the substrate and spaced from the mass.
 14. The apparatusof claim 8, wherein the mass comprises a surface that faces thesubstrate, the apparatus further comprising: a mass stop extending fromthe surface of the mass and spaced from the substrate.
 15. The apparatusof claim 8 wherein: the anti-stiction element has a second end protrudesbeyond the side of the mass.
 16. The apparatus of claim 8 wherein themass includes a plate that is substantially parallel to a portion of thesubstrate.
 17. The apparatus according to claim 8, wherein the firstelastic element includes a torsional spring coupled to and extendingbetween the mass and substrate.